1. Field of the Invention
The invention relates to multiple emulsions, to their production, and to their use.
2. Background Art
Emulsions require the immiscibility of two phases. Emulsions can therefore be formed if two phases, phase A and phase B, are not completely miscible with one another. Typically, phase A is an aqueous phase, and phase B is an oily phase. Accordingly, it is possible to produce water-in-oil (w/o) and oil-in-water (o/w) emulsions.
Conventional emulsions consist of a water phase and an oil phase and one or more surface-active substances. Typically, surface-active substances are low molecular weight substances which contain one or more polar groups and also contain one or more nonpolar groups. Emulsions which are stabilized by means of particles were described for the first time by Pickering: J. CHEM. Soc., 1907, 91, 2001.
For some applications, multiphasic emulsions (“multiple emulsions”) are desirable. The first experiments for the preparation of multiple emulsions were described by Matsumoto et al., J. COLL. INTERF. SCI., 1976, 57, 353. A problem with multiple emulsions is their instability toward coalescence.
Multiple emulsions using polymeric surfactants have been described in DE 41 36699 A1 and DE 33 39 056 C2. The processes disclosed, however, are complex, costly, perturbation-sensitive, and lead to high-viscosity systems. Multiple emulsions using gelling agents in the internal phase have been described in DE 196 38 729 A1. The process disclosed is based on imparting thixotropy to and thickening of the internal aqueous phase, which significantly restricts its applicability.
Multiple emulsions consist of    (1) a phase X, which is present as an emulsion in a    (2) phase Y, this phase X-in-phase Y emulsion (X/Y) being present    (3) in turn as droplets in a further phase Z as an emulsion:    Phase X-in-phase Y-in-phase Z (Phase X/Phase Y/Phase Z).
In practice, two principal possibilities for multiple emulsions arise from this:
Possibility I: (A/B/A′).
Multiple emulsions consisting of (1) a phase A which is present in emulsified form in a (2) phase B, this phase A-in-phase B emulsion (A/B) is present (3) in turn as droplets in a further phase A′: phase A-in-phase B-in-phase A′ (A/B/A′).
Possibility II: (B/A/B′).
Multiple emulsions consisting of (1) a phase B which is present in emulsified form in a (2) phase A, this phase B-in-phase A emulsion (B/A) is present (3) in turn as droplets in a further phase B′: phase B-in-phase A-in-phase B′ (B/A/B′).
In practice, with phase A as the aqueous phase and phase B as the oily phase, it is possible to prepare multiple emulsions in the form of    (I) water-in-oil-in-water emulsion (w/o/w)    or in the form of    (II) oil-in-water-in-oil emulsion (o/w/o).
The stabilization of multiple emulsions known hitherto requires the use of low molecular weight oligomeric or polymeric emulsifiers or surface-active substances which stabilize the various w/o and o/w interfaces. However, multiple emulsions known hitherto suffer from coalescence and sedimentation, lack shear stability, and possess inadequate storage stability, leading to loss of their properties, lending them unsuitable for their intended applications.
It has hitherto been impossible to prepare multiple emulsions having long-term stability with feasible expenditure. For example, to date, no simple and universal process has been described which facilitates the preparation of multiple emulsions with long-term storage stability and shear stability.
Multiple emulsions are of great technical interest. They are, for example, of key technical importance in applications which require a “controlled release” function. Controlled release means the controlled and slow release of (I) active ingredients dissolved in polar aqueous phase or (II) active ingredients dissolved in nonpolar oily phase, and is of great interest in a wide variety of applications in pharmacy, medicine, agrochemistry, foodstuffs and animal feeds, in cosmetics, and in chemical catalysis.